RADIO CONSTRAINTS ON CORONAL MODELS FOR DME STARS

Radio data are used to test coronal models for dMe stars. Specifically , we show that pnotospheric magnetic field observations imply that the low corona of a dMe star should be saturated by magnetic fields with an average strength in excess of 1 kG. In such fields the hot componen t of the corona detected in X-ray observations (temperature of order 2 x 10(7) K) would be optically thick at least up to 15 GHz due to ther mal gyroresonance opacity. The resulting emission would easily be dete ctable by radio observations and should have a radio spectrum rising i n the microwave range. We have carried out observations to test this p rediction, and in the majority of cases find that the observed fluxes at 15 GHz are too low to be consistent with the assumptions. In the fe w cases where the stars were detected at 15 GHz, the evidence indicate s that the observed emission is nonthermal. These results imply that t he hot component of the X-ray-emitting plasma in the corona is not coi ncident with the strong magnetic fields in the lower corona. Because t he hot plasma must still be confined by closed magnetic held lines, it is likely to be restricted to heights of the order of a stellar radiu s above the photosphere. The results seem to imply a different genesis for the two components of the X-ray-emitting corona of flare stars: t he hot component may be cooling flare plasma, while the cooler compone nt (temperature of order 3 x 10(6) K) is associated with a more conven tional coronal heating mechanism.